/*
 * Copyright 2002-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http.codec;

import org.reactivestreams.Publisher;
import org.springframework.core.ResolvableType;
import org.springframework.core.codec.Hints;
import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.log.LogFormatUtils;
import org.springframework.http.MediaType;
import org.springframework.http.ReactiveHttpOutputMessage;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.MultiValueMap;
import reactor.core.publisher.Mono;

import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.util.Collections;
import java.util.List;
import java.util.Map;

/**
 * {@link HttpMessageWriter} for writing a {@code MultiValueMap<String, String>}
 * as HTML form data, i.e. {@code "application/x-www-form-urlencoded"}, to the
 * body of a request.
 *
 * <p>Note that unless the media type is explicitly set to
 * {@link MediaType#APPLICATION_FORM_URLENCODED}, the {@link #canWrite} method
 * will need generic type information to confirm the target map has String values.
 * This is because a MultiValueMap with non-String values can be used to write
 * multipart requests.
 *
 * <p>To support both form data and multipart requests, consider using
 * {@link org.springframework.http.codec.multipart.MultipartHttpMessageWriter}
 * configured with this writer as the fallback for writing plain form data.
 *
 * @author Sebastien Deleuze
 * @author Rossen Stoyanchev
 * @see org.springframework.http.codec.multipart.MultipartHttpMessageWriter
 * @since 5.0
 */
public class FormHttpMessageWriter extends LoggingCodecSupport
        implements HttpMessageWriter<MultiValueMap<String, String>> {

    /**
     * The default charset used by the writer.
     */
    public static final Charset DEFAULT_CHARSET = StandardCharsets.UTF_8;

    private static final MediaType DEFAULT_FORM_DATA_MEDIA_TYPE =
            new MediaType(MediaType.APPLICATION_FORM_URLENCODED, DEFAULT_CHARSET);

    private static final List<MediaType> MEDIA_TYPES =
            Collections.singletonList(MediaType.APPLICATION_FORM_URLENCODED);

    private static final ResolvableType MULTIVALUE_TYPE =
            ResolvableType.forClassWithGenerics(MultiValueMap.class, String.class, String.class);


    private Charset defaultCharset = DEFAULT_CHARSET;

    /**
     * Return the configured default charset.
     */
    public Charset getDefaultCharset() {
        return this.defaultCharset;
    }

    /**
     * Set the default character set to use for writing form data when the response
     * Content-Type header does not explicitly specify it.
     * <p>By default this is set to "UTF-8".
     */
    public void setDefaultCharset(Charset charset) {
        Assert.notNull(charset, "Charset must not be null");
        this.defaultCharset = charset;
    }

    @Override
    public List<MediaType> getWritableMediaTypes() {
        return MEDIA_TYPES;
    }


    @Override
    public boolean canWrite(ResolvableType elementType, @Nullable MediaType mediaType) {
        if (!MultiValueMap.class.isAssignableFrom(elementType.toClass())) {
            return false;
        }
        if (MediaType.APPLICATION_FORM_URLENCODED.isCompatibleWith(mediaType)) {
            // Optimistically, any MultiValueMap with or without generics
            return true;
        }
        if (mediaType == null) {
            // Only String-based MultiValueMap
            return MULTIVALUE_TYPE.isAssignableFrom(elementType);
        }
        return false;
    }

    @Override
    public Mono<Void> write(Publisher<? extends MultiValueMap<String, String>> inputStream,
                            ResolvableType elementType, @Nullable MediaType mediaType, ReactiveHttpOutputMessage message,
                            Map<String, Object> hints) {

        mediaType = getMediaType(mediaType);
        message.getHeaders().setContentType(mediaType);

        Charset charset = mediaType.getCharset();
        Assert.notNull(charset, "No charset"); // should never occur

        return Mono.from(inputStream).flatMap(form -> {
            logFormData(form, hints);
            String value = serializeForm(form, charset);
            ByteBuffer byteBuffer = charset.encode(value);
            DataBuffer buffer = message.bufferFactory().wrap(byteBuffer); // wrapping only, no allocation
            message.getHeaders().setContentLength(byteBuffer.remaining());
            return message.writeWith(Mono.just(buffer));
        });
    }

    private MediaType getMediaType(@Nullable MediaType mediaType) {
        if (mediaType == null) {
            return DEFAULT_FORM_DATA_MEDIA_TYPE;
        } else if (mediaType.getCharset() == null) {
            return new MediaType(mediaType, getDefaultCharset());
        } else {
            return mediaType;
        }
    }

    private void logFormData(MultiValueMap<String, String> form, Map<String, Object> hints) {
        LogFormatUtils.traceDebug(logger, traceOn -> Hints.getLogPrefix(hints) + "Writing " +
                (isEnableLoggingRequestDetails() ?
                        LogFormatUtils.formatValue(form, !traceOn) :
                        "form fields " + form.keySet() + " (content masked)"));
    }

    protected String serializeForm(MultiValueMap<String, String> formData, Charset charset) {
        StringBuilder builder = new StringBuilder();
        formData.forEach((name, values) ->
                values.forEach(value -> {
                    try {
                        if (builder.length() != 0) {
                            builder.append('&');
                        }
                        builder.append(URLEncoder.encode(name, charset.name()));
                        if (value != null) {
                            builder.append('=');
                            builder.append(URLEncoder.encode(value, charset.name()));
                        }
                    } catch (UnsupportedEncodingException ex) {
                        throw new IllegalStateException(ex);
                    }
                }));
        return builder.toString();
    }

}
